Tip of the day...
Most piston and ring manufacturing company's use OEM standards to make parts fit.
If Yamaha states the bore is 64.00, the piston and ring parts are made to fit in that size of bore.
When bored oversized lets say 65.50mm the piston clearance is built into the piston sizing, so a cast Pro-X may have .0018 clearance, but a
Wiseco forged piston will measure .003 clearance in the same bore. A forged piston will need more piston the wall clearance to expand.
This way Piston Ring tolerances can be followed! IMPORTANT!
For every .001in bore size larger, the ring end gaps will get bigger times PI, which is .00314
I have found through 35 years of building RD aircooled engines I do set the piston to wall clearance for the job, type of piston, what bore the piston is and what brand piston is. OEM Yamaha, Pro-X(Banshee), Wiseco, and Wossner are all different piston clearances.
Example....If I bore a stock RD 350/400 1st over @ 64.25mm with a Pro-X banshee piston and the bike is stock I'll set piston to wall @ .0019 for today's fuel. (never follow stock spec's unless your going to use canned race fuel)
If the RD is a hotrod, cut heads, aftermarket heads with pipes I'll be .0022
If the same mods but at 65.75mm bore I'll set clearances to .0025 for the thicker pistons that will need more running clearance.
Wiseco and Wosser pistons will follow suit, but using greater clearances due to being forged pistons.
No need for a Wiseco/Wossner forged piston in a stock bike unless a race bike like a Historic Production Class that will have the snot ran out out it for everything it will give you...
stock and update porting engines builds Cast Pro-X are fine.
Most of my stg 1 builds it's optional on cast or forged pistons, with stg 2 and up to full race porting, I only use forged pistons and even do
piston coatings to strengthen them of to be more fool proof ( like opp's I missed the jetting this morning and didn't hurt it in practice)
In Guillermo's setup @ 66.25mm bore, using uncoated pistons, it's perfectly happy at .0035 with a good warm up before a taxing run.
Chuck
http://www.2strokeworld.net/forum/index.php?topic=1720.0 (http://www.2strokeworld.net/forum/index.php?topic=1720.0)
Hi Chuck! Thanks for all the tech tips you post here, they're hugely helpful to me!
I'm building an aircooled RD350 for road racing in USCRA. The cylinders are heavily ported, and heads have had some squish band work done. I have Wiseco pistons measuring about 65.44mm (2.5760" to be a little more exact).
What do you recommend for a clearance for this type of application? How much clearance is excessive?
I'm measuring about 0.05mm (0.002") Clearance, and I'm worried that's far too much. I also admit my measurement skills are rudimentary, and I may need more practice to get a better reading.
Quote from: bostontrackrat on May 20, 2020, 12:09:07 PM
Hi Chuck! Thanks for all the tech tips you post here, they're hugely helpful to me!
I'm building an aircooled RD350 for road racing in USCRA. The cylinders are heavily ported, and heads have had some squish band work done. I have Wiseco pistons measuring about 65.44mm (2.5760" to be a little more exact).
What do you recommend for a clearance for this type of application? How much clearance is excessive?
I'm measuring about 0.05mm (0.002") Clearance, and I'm worried that's far too much. I also admit my measurement skills are rudimentary, and I may need more practice to get a better reading.
No .002 is way too tight!! :eek:
Can you tell me if your using 393 RD pistons or the Banshee 513 pistons?
No less than .0032. with a break in. I run Wossners @ .0035.
I have gone as little as .0027 with ceramic coated Wiseco pistons @ 20:1 premix and always use at least 110* octane race fuel.
Chuck
The Wiseco's will go away in 5 or 6 race double header weekends. When they get to about .007-.008 they're done for.
C
Thanks for the info Chuck! I just chatted with a friend of mine who runs a shop and he advised me that I've made a pretty good error trying to take these measurements with regular ol' calipers to measure the telescoping gauges. I just ran out and picked up a set of micrometers. I'll remeasure tonight or tomorrow and let you know if I find different numbers.
They are 315 banshee pistons. (that reminds me, I need to file the center tab off still.)
I went to Harbor Freight today (I know, I know) and grabbed a set of micrometers to go with the telescoping bore gauges I bought earlier this week.
New measurements are WAY more repeatable, and far more worrisome.
Cylinder 1: 2.577"
Cylinder 2: 2.577"
Piston 1: 2.576"
Piston 2: 2.576"
I took the average of 4 measurements for each one. All measurements were within 1 thou for each part.
I think I'm going to need to have the bored widened out a little more....
I measured the cylinders at both the top and the bottom, they were the same. I'm taking the measurements for the pistons at the bottom of the skirt, this is what I read online, and in the 2 Stroke Tuner's Handbook. Right?
(https://i.imgur.com/NPnPrl3.jpg)
Borrowed a hone from a friend and have been able to make a clearance of 3 thousandths. :toot:
(https://i.imgur.com/So9LNza.jpg)
No good machine shops around? Maybe all closed up there?
That cylinder hone just burned my eyes out... :eek:
Not a very good one for doing this kind of work. That hone is just for breaking a glaze off a wall, not stock removal.
I guess it's time I get some new honing stuff up...
Pic's of what I use.
(https://photos.imageevent.com/supertune/rddocumentstuff/large/P5200122.jpg)
(https://photos.imageevent.com/supertune/rddocumentstuff/large/dsc03509_261.jpg)
(https://photos.imageevent.com/supertune/rddocumentstuff/large/s-l640.jpg)
Chuck
Yeahhhhh..... I should probably call a few shops on Tuesday. Damn shame, I was really really looking forward to getting the engine back into the bike this weekend. :bang: